230 WILLIAM S. COOPER 



nance in the order given above, because of differences in rapidity 

 of development, the smallest plants first, then the shrubs, and 

 finally the trees. The period of dominance for the first and 

 second stages does not coincide with the period of most perfect 

 development. The Dryas-Ardostaphylos Community is the 

 dominant community upon Moraine III because the others are 

 rudimentary. It is a much more solidly developed community 

 in the spots where it occurs upon the older moraines, but upon 

 these it is losing ground in competition with the shrubs and 

 trees. The Salix-Betula Community, clearly dominant upon 

 Moraine II, is found in its best development upon the oldest 

 moraine, but is here overtopped and doomed to supersession by 

 the Climax Forest, 



4. The Shingle Flat Succession 



The shingle flat is one of the most characteristic features of 

 glaciated mountain regions. Morainic material is being con- 

 stantly carried from its place of deposition by the torrential 

 streams from the glaciers. Much fine silt is borne by the streams 

 themselves as they issue from beneath the ice. All these mate- 

 rials are carried for greater or less distances, and in deposition are 

 roughly sorted through differences in velocity and volume of the 

 flowing water, so that a deposit in a given spot is likely to be 

 rather uniform in size of component materials. Wherever 

 conditions are such that a stream becomes overloaded we find 

 shingle or silt flats. The characteristic grade profile of a glaci- 

 ated valley with its long stretches of gentle gradient separated 

 by step-like cliffs gives ample opportunity for the deposition of 

 stream-borne materials; so that a river valley below a glacier 

 is likely to be a more or less interrupted flood plain, with numer- 

 ous meandering and anastomosing streams flowing upon it. 

 The abundance of lakes in glaciated regions, with the consequent 

 slackening of current of the inflowing streams, also favors the 

 formation of alluvial flats. The square-cut upper ends of many 

 alpine lakes are a familiar consequence of this. The present 

 study covered a fine series of shingle flats along the Grand Fork 



